The present invention relates to a level conversion circuit for effecting signal level conversion between two devices each being operable with a particular power source voltage. More particularly, the present invention is concerned with a level conversion circuit for connecting devices respectively operable with a power source voltage of 3.3 V and a power source voltage of 2.5 V.
Japanese Patent Laid-Open Publication No. 9-34608, for example, discloses an interface circuit which is a specific form of a level conversion circuit. The interface circuit includes a voltage monitoring circuit for determining whether or not the power source voltage of a first circuit or that of a second circuit operable with a different voltage from the first circuit has exceeded a preselected voltage. In response to the result of this decision, a signal switching circuit selects either one of a first and a second signal path. As a result, a signal is input and output via the first signal path or a level conversion circuit included in the second signal path. This implements signal transfer between the first and second circuits when the power source voltage does not exceed the preselected voltage.
The above conventional interface circuit is able to convert a low power source voltage to a high power source voltage, but unable to convert a signal level such that, e.g., a signal output from a 3.3 V device operable with a power source voltage of 3.3 V can be input to a 2.5 V device operable with a lower power source voltage of 2.5 V. Should the 3.3 V device be directly connected to the 2.5 V device, a piercing current would flow and damage the 2.5 V device.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 57-19839, 58-12042, 61-272820, 63-216127, 64-76211, 1-298410, 2-211517, 4-143820, and 8-286795.